THIS INVENTION relates to the removal of impurities from a hydrocarbon component or fraction. In particular, it relates to a process for removing impurities from a liquid hydrocarbon component or fraction.
According to the invention, there is provided a process for removing impurities from the hydrocarbon component or fraction, which process comprises
mixing, in a liquid-liquid extraction step, an impurity-containing liquid hydrocarbon component or fraction, as an impure liquid hydrocarbon feedstock, with an acetonitrile-based solvent, thereby to extract at least one impurity from the hydrocarbon component or fraction into the solvent;
withdrawing from the extraction step, as a raffinate, purified hydrocarbon component or fraction; and
withdrawing from the extraction step, as an extract, impurity-containing solvent.
When a hydrocarbon component or fraction, ie a hydrocarbon feedstock, is worked up to obtain particular products therefrom, impurities present in the hydrocarbon feedstock can adversely affect the quality and purity of the products obtained, can cause catalyst poisoning and increase catalyst consumption when the working up involves catalytic treatment of the hydrocarbon feedstock, and can cause unwanted side reactions to form during such work-up. The process of the present invention thus provides a means of purifying such an impure liquid hydrocarbon feedstock, prior to subjecting it to the further work-up, so that the problems associated with such work-up of the hydrocarbon feedstock, are at least reduced.
The hydrocarbon feedstock may be an olefinic and/or naphthenic hydrocarbon feedstock, which may contain at least 20% (by mass) olefins and/or naphthenes. The olefins and/or naphthenes may typically contain from 8 to 14 carbon atoms, ie the feedstock may be a C8 to C14 olefinic and/or naphthenic feedstock. Instead, for example, the feedstock may comprise a narrower cut of olefins and/or naphthenes, eg it may be a C8 to C10, a C10, a C11/12 or a C13/14 olefinic and/or naphthenic feedstock.
The hydrocarbon feedstock may, in particular, be Fischer-Tropsch derived. By xe2x80x98Fischer-Tropsch derivedxe2x80x99 is meant a mixture, component or fraction obtained by subjecting a synthesis gas comprising carbon monoxide and hydrogen to Fischer-Tropsch reaction conditions in the presence of an iron-based Fischer-Tropsch catalyst, a cobalt based Fischer-Tropsch catalyst, an iron/cobalt based Fischer-Tropsch catalyst, or a mixture of two or more of such Fischer-Tropsch catalysts, with the resultant Fischer-Tropsch reaction products being worked up to obtain the mixture, component or fraction in question.
The impurity or impurities present in the hydrocarbon feedstock may be at least one carboxylic acid, oxygenate, phenol, aromatic compound and/or cyclic compound. At least one of these impurities will thus be removed from the feedstock in the liquid-liquid extraction step.
Typically, the hydrocarbon feedstock may comprise, on a mass basis, 40%-60% olefins, 10%-30% paraffins, 5%-30% oxygenates such as alcohols, ketones and/or esters, 0.5%-1% phenols and/or cresols, 1%-6% carboxylic acids, and 5%-30% aromatic compounds.
While the solvent can, at least in principle, be pure acetonitrile which is immiscible with the hydrocarbon component, it will usually comprise a mixture or solution of acetonitrile and water. The water content of the solvent will be determined by factors such as the required selectivity and capacity of the solvent, the ease of operation of the extraction stage, the cost of subsequent solvent recovery, and the method used to control the water balance in the solvent. Thus, the water concentration in the acetonitrile-based solvent, on a mass basis, may, for a C8-C10 olefinic and/or naphthenic feedstock, be between 10% and 20%, preferably about 15%; for a C11/12 olefinic and/or naphthenic feedstock between 15% and 35%, preferably about 20%; and for a C13/14 olefinic and/or naphthenic feedstock between 20% and 35%, preferably about 25%.
The solvent to hydrocarbon component or feedstock ratio is determined by the degree of impurity removal required, and by the impurity species which it is desired to remove. In other words, it has surprisingly been found that by selecting the appropriate solvent to feedstock ratio, the impurity which is removed can be selected. Thus, for example, for almost complete removal of carboxylic acid impurities, the mass ratio of solvent to hydrocarbon feedstock may be between 0.3:1 and 2:1, typically about 0.5:1. However, for removal of carboxylic acid, oxygenate and aromatic impurities, the mass ratio of solvent to hydrocarbon feedstock may be between 1:1 and 8:1, typically about 6:1. Thus, at low solvent to feedstock ratios, virtually only carboxylic acids will be removed; at intermediate solvent to feedstock ratios, oxygenates will also be removed; and at high solvent to feedstock ratios, carboxylic acids, oxygenates and aromatics will be removed.
The liquid-liquid extraction step may, in particular, comprise counter-current extraction in which a continuous stream of the hydrocarbon feedstock passes in counter-current fashion to a continuous stream of the solvent. The extraction may, in particular, be effected in a multi-stage liquid-liquid extraction column or extractor, with the feedstock entering the column near its bottom, the solvent entering the column near its top, the raffinate being withdrawn at the top of the column, and the extract being withdrawn at the bottom of the column. The extraction column may operate at about ambient pressure or higher, eg up to a maximum of about 10 bar(a), and at about ambient temperature or higher, eg at between 30xc2x0 C and 150xc2x0 C.
The raffinate will normally contain some solvent, in addition to the purified hydrocarbon feedstock. The process may thus include, in a raffinate stripping step, separating solvent from the purified hydrocarbon feedstock. The raffinate stripping may typically be effected in a multi-stage stripper column with solvent being withdrawn from the top of the column and being recycled to the extraction step, and purified hydrocarbon feedstock being withdrawn from the bottom thereof.
For a C8-C10, olefinic and/or naphthenic feedstock, the raffinate stripper column may operate at above atmospheric pressure, eg at about 1.5 bar(a); however, for C10-C14 olefin and/or naphthenic feedstock, the pressure may vary from below atmospheric pressure to above atmospheric pressure, eg the operating pressure may then be between 0.1 bar(a) and 1.5 bar(a). The actual operating pressure will be determined by the maximum allowable bottom temperature in the column, since the purified hydrocarbon feedstock will usually be heat-sensitive.
The raffinate may be preheated before entering the stripper column, eg preheated to about 60xc2x0 C.
If desired, water may be added to the raffinate stripper column, preferably below the hydrocarbon feedstock entry point. The water is then preferably preheated, eg to about 80xc2x0 C. The process may then include withdrawing a bottoms product from the raffinate stripper column, and, in a phase separation step, separating the bottoms product into an aqueous phase and purified hydrocarbon feedstock or raffinate, with the aqueous phase being returned to the raffinate stripping column. Make-up water can then be added to the phase separation step for water balance. The water addition option will normally be used for a C11-C14 olefinic and/or naphthenic feedstock, to avoid having to operate the column under vacuum, which would require the use of a chiller unit to accommodate low overhead condensing temperatures, and larger equipment.
The extract from the liquid-liquid extraction step will contain, in addition to the solvent, also the extracted impurity or impurities, and, usually, some co-extracted hydrocarbons. The process may thus include, in an extract stripping step, separating the solvent from the impurity and the hydrocarbons, ie from an impurity/hydrocarbon mixture. The extract stripping may also be effected in a multi-stage stripper column, with solvent being withdrawn from the top of the column and being recycled to the extraction step, and the impurity/hydrocarbon mixture being withdrawn from the bottom thereof. With a C8 to C11 feedstock, co-extracted hydrocarbons are usually recovered overhead with the solvent.
The extract may be preheated, eg to about 60 xe2x96xa1 C., before entering the extract stripper column. The column is preferably operated at above atmospheric pressure, eg at a pressure up to about 1.5 atm(a) or higher. If desired, water can be added to the extract stripper column in similar fashion as hereinbefore described in respect of the raffinate stripper column. The water, when used, will normally be preheated, eg to about 80xe2x96xa1 C. The process may then include withdrawing a bottoms product from the extract stripper column; and, in a phase separation step, separating the bottoms product into an aqueous phase and the impurity/hydrocarbon mixture. The aqueous phase may then partially be recycled to the extract stripper column, and partially purged to achieve a water balance.
The overheads or recovered solvent from both stripper columns may thus be recycled to the extraction step. A water balance is ensured in the process by either using a membrane separation process, as a first mode of the operation, or by purging excess water from the bottom of the extract stripper column, as a second mode of the operation. The optimum operation is dependant on the composition of the feed material. When the membrane separation process is used, then the overheads or recovered solvent from either one or both the stripper columns is passed through a suitable membrane to separate water therefrom.